Aerosols their Direct and Indirect Climate Effects

Although greenhouse gases and cloud droplets and cloud ice are dominant constituents of the atmosphere determining to a large extent the radiation budget of the planet Earth, also aerosol2 particles play a major role for our planet. These tiny particles suspended in air in the size range from about 1 nanometer (nm) to about 10 micrometers (^m) radius are either emitted from the Earth's surface or form in the atmosphere from precursor gases like sulphur dioxide (SO2). While the small ones (< 0.01 ^m radius) often get attached to other larger particles or surfaces by the molecules' Brownian motion, the larger ones, called coarse particles, with r > 1^m, settle through gravity. Therefore, maximum spectral concentration (particles per unit volume per unit of radius) is often close to r = 0.01 ^m and typical lifetime in the free troposphere reaches weeks for particles in the size range around 0.1 ^m. Their main sink process is activation as a cloud condensation nucleus and subsequent rain-out, and much less below cloud scavenging. As any cloud droplet needs an aerosol particle as a condensation nucleus, it is clear that the aerosol particles can strongly influence the optical properties of clouds and can exert an indirect effect besides the direct one, which anybody can see through atmospheric turbidity, the result of scattering of visible light by aerosol particles.

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